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MongoDB Aggregation Pipeline Lookup | About

The Aggregation Pipeline in MongoDB is a great tool that helps with handling and analyzing data stored in collections.

Among the main stages within this pipeline, the $lookup step is a game-changer. It helps with the left outer joins between two collections.

Today, we are going to take a closer look at the $lookup stage, its applications, and its potential.

The $lookup stage is the go-to tool for enriching documents. It combines a collection with reference data or generates extensive reports by combining data from several collections.

Here are the parameters of the $lookup stage:

• `from`: The name of the collection to be merged with the existing one.
• `localField`: The field in the current collection serving as the join key.
• `foreignField`: The field in the “from” collection serving as the join key.
• `as`: The name of the new array field in the result document, encapsulating the outcomes of the join.

Syntax for $lookup Stage

{
$lookup:
{
from: <collection to join>,
localField: <field from the input documents>,
foreignField: <field from the documents of the "from" collection>,
as: <output array field>
}
}


Here is an example to understand how the $lookup stage. . In this case, we are working with the “sample_mflix” database and the “comments” collection. The goal is to enrich the comments with movie details through a left outer join with the “movies” collection.


db.orders.aggregate([
{
$lookup: {
from: "customers",
localField: "customer_id",
foreignField: "_id",
as: "customer_info",
},
},
{
$match: {
"customer_info": { $ne: [] } // Exclude orders without matching customers
}
},
{
$limit: 3
}
])


In the above example, we are working with an “orders” collection and want to enrich each order document with information about the corresponding customer.

The $lookup stage performs a left outer join with the “customers” collection based on the “customer_id” field, creating a new array field “customer_info” in the result document.

The $match stage ensures that only orders with matching customers are included, and the $limit stage limits the output to the first three enriched orders.

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Conclusion

In brief, our Support Experts introduced us to MongoDB Aggregation Pipeline Lookup.